The zoanthid coral genus that contains species associated with the potent neurotoxin palytoxin is Palythoa. Palytoxin has been detected in many, though not all, members of this genus, so the answer identifies the correct genus rather than claiming that every individual colony is toxic.
Palythoa species are colonial anthozoans whose polyps are partly embedded in a tough, encrusting mat called coenenchyme. Their colonies may be brown, cream, yellow, white, or green, and they are common in marine aquariums. The species Palythoa toxica, native to Hawaii, is especially famous because palytoxin was first isolated from it in 1971.
The toxin is dangerous because it can severely disrupt cell membranes and cause serious poisoning through contact, inhalation of aerosols, or accidental ingestion. Aquarium keepers are therefore advised to use gloves and eye protection, particularly when handling or heating colonies.
A frequent confusion is between Palythoa and Zoanthus, both popular zoanthid genera. They can resemble each other, but the classic palytoxin association in trivia and aquarium safety discussions is Palythoa. Research also suggests some toxin may originate from associated microorganisms rather than the coral itself.